The present invention relates to a combined pneumatic-hydraulic press with controlled stroke.
Combined pneumatic-hydraulic presses are known which generally comprise a head constituted by a substantially cylindrical body in which a cylindrical main chamber is defined; said main chamber accommodates a working piston so that it can slide axially, and the stem of said piston protrudes from one side of the head. This stem constitutes the element of the press which, associated with various types of tools, performs the various treatments for which presses are normally used, such as clamping, blanking, marking, straightening, riveting, calking, coining, bending, drawing, keying, etc.
The main chamber can be selectively connected, on opposite sides with respect to the working piston, to a source of compressed air or to the atmosphere so as to rapidly move, with a reduced force, the stem toward or away from the working surface on which the part to be treated is arranged.
The working piston is generally provided with a wing which extends on the opposite side with respect to the stem and can slide in a chamber which is filled with liquid. Said liquid-containing chamber has a narrower portion in which the stem of a pneumatically-actuated piston can slide sealingly. The assembly constituted by the liquid-containing chamber, by the wing of the working piston and by the stem of said pneumatically-actuated piston constitutes a hydraulic press in which the actuation force of the stem which slides in the narrower portion of the liquid-containing chamber is multiplied and transmitted to the working piston in order to obtain an adequate force during the working step.
These types of presses with combined pneumatic-hydraulic actuation, despite having undeniable advantages with respect to presses with exclusively hydraulic actuation, have some problems.
Since the working piston is conceived like the piston of a double-action pneumatic cylinder, during the rapid approach step said working piston in fact draws the liquid contained in the chamber in which the wing which constitutes an element of the hydraulic press slides. This drawing action, allowed by a compensation element which sends liquid into said chamber or keeps its volume constant by means of another sliding piston which delimits the chamber on the side opposite to said wing, exerts a braking effect on the working piston, reducing its speed of approach to the part to be treated or requiring greater pneumatic power in order to obtain the required speed. Furthermore, the filling of the chamber with liquid can be incomplete, causing operating anomalies, in case of high actuation speeds.
Another disadvantage which can be observed in presses with combined pneumatic-hydraulic actuation is that it is not possible to precisely control the stroke of the working piston. Due to the way in which currently commercially available combined pneumatic-hydraulic presses are structured, the stroke of the working piston can in fact be controlled only indirectly and with scarce precision by controlling the stroke of the pneumatically-actuated piston.
Due to this fact, currently commercially available combined pneumatic-hydraulic presses cannot be used to actuate machines, such as for example rolling machines, machines for inserting dowels, rivets etc., which have a controlled amount of penetration and require an extremely precise working piston stroke; due to this reason, these types of machines are currently actuated by means of hydraulic or pneumatic actuators equipped with levers for controlling the stroke of the working piston.